1. Field of the Invention
This invention relates generally to neutral colored glass compositions and, more particularly, to gray colored glass compositions having low transmitted color shift characteristics which are particularly well suited for automotive vision panels, such as windshields and front sidelights.
2. Technical Considerations
In different parts of the world, governmental agencies with responsibility for regulating or licensing motor vehicle safety or use of highways or other public thoroughfares have prescribed minimum luminous light transmittance values for particular automotive “vision panels”, such as windshields and front sidelights. For instance, United States Federal regulations require the luminous light transmittance (LTA) of automotive windshields and front sidelights to be at least 70%. The luminous transmittance requirements for other automotive transparencies, such as back sidelights and rear lights of trucks and minivans, and for non-vision panels, such as sun roofs, moon roofs and the like, are typically less than those for windshields and front sidelights. Other areas of the world may have a different prescribed minimum.
Colored or coated automotive transparencies in use today which meet the mandated luminous transmission requirements may also provide some degree of shading or solar control properties, for example, to help decrease the detrimental effects of ultraviolet radiation on the interior of the vehicle, e.g., fabric fading. However, while these known automotive transparencies may provide some degree of solar energy protection, they also tend to have an effect on the perceived color of an object viewed through the transparency. For example, the color of a vehicle interior, e.g., the interior fabric color, perceived from outside the vehicle seen through a conventional colored automotive transparency may appear to be of a different color than the actual color of the interior. If the vehicle interior has been selected to provide a certain aesthetic effect with respect to the overall appearance of the vehicle, this perceived or “transmitted color shift” may adversely impact upon the overall aesthetic appearance of the vehicle.
Therefore, a neutral colored glass, e.g., a glass having a lower excitation purity or a less intense color, such as gray, which reduces this perceived color shift while also providing good solar performance properties would be advantageous. However, forming such a glass poses various production concerns. For example, most colored automotive glass compositions having good solar control properties, such as infrared (“IR”) or ultraviolet (“UV”) absorption and/or reflection, also have a moderate to high concentration of ferrous iron (FeO). Ferrous iron produces a broad absorption band in the red to near IR region of the solar spectrum. The concentration of ferrous iron in glass depends on both the total concentration of iron oxide and the oxidation state of glass, or its redox ratio. Therefore, achieving moderate to higher levels of ferrous iron in glass can involve increasing either the total iron concentration of the glass or the glass redox ratio, or both.
At a commonly practiced redox ratio of 0.35 or less, increasing total iron in the glass generally results in green colors. On the other hand, increasing the glass redox ratio shifts the glass color to blue. Increasing either or both of these variables also can result in lower luminous light transmittance (LTA) due to higher absorption of visible light. Thus, to achieve high IR absorption in a neutral colored glass while maintaining a high level of visible transmittance to meet mandated minimum LTA regulations is particularly difficult.
Therefore, it is an object of the present invention to provide glass compositions and automotive vision panels which are neutral in color, which provide good solar performance properties, and which also provide lower transmitted color shift characteristics than conventional glass compositions. The glass compositions of the invention can be produced over a wide range of redox ratios.